1. Field of the Invention
Embodiments of the present invention generally relate to method and apparatus for processing a semiconductor substrate. More particularly, embodiments of the present invention provide method and apparatus for correcting baseline skew in an inductively coupled plasma reactor.
2. Description of the Related Art
Plasma reactors used to fabricate semiconductor microelectronic circuits can employ RF (radio frequency) inductively coupled fields to maintain a plasma formed from a processing gas. Conventional inductively coupled plasma reactors generally includes a vacuum chamber having a side wall and a ceiling, a workpiece support pedestal within the chamber and generally facing the ceiling, a gas inlet capable of supplying a process gas into the chamber, and one or more solenoidal interleaved parallel conductor coil antennas overlying the ceiling. The one or more coil antennas are generally wound about an axis of symmetry generally perpendicular to the ceiling. A RF plasma source power supply is connected across each of the coil antennas. Sometimes, the reactor may include an inner coil antenna overlying the ceiling and surrounded by an outer coil antenna.
Typically, a high frequency RF source power signal is applied to the one or more coil antennas near the reactor chamber ceiling. A substrate disposed on a pedestal within the chamber which may have a bias RF signal applied to it. The power of the signal applied to the coil antenna primarily determines the plasma ion density within the chamber, while the power of the bias signal applied to the substrate determines the ion energy at the wafer surface.
Typically with “inner” and “outer” coil antennas, the coils are distributed radially or horizontally (rather than being confined to a discrete radius) so that their radial location is diffused accordingly. The radial distribution of plasma ion distribution is changed by changing the relative apportionment of applied RF power between the inner and outer antennas. However, it becomes more difficult to maintain a uniform plasma ion density across the entire wafer surface as wafers become larger and device feature become smaller.
FIG. 1 schematically illustrates non-uniformity problems cased by baseline skew encountered by typical inductively coupled plasma reactors. FIG. 1 is a result showing nitrogen dosages across a substrate after nitridation processes preformed in a typical inductively coupled plasma reactor. The nitridation processes are performed to silicon dioxide gate dielectric film formed on a substrate. The substrate is positioned in a vacuum chamber capable of generating inductively coupled plasma. Nitrogen gas is flown to the plasma chamber and a plasma is struck while the flow continuous. The nitrogen radicals and/or nitrogen ions in the nitrogen plasma then diffuse and/or bombard into the silicon dioxide gate dielectric film.
FIG. 1 is a contour graph showing nitrogen dosage across surface of an entire surface of a 300 mm substrate after nitridation performed in an inductively coupled plasma reactor. The asymmetrical distribution of nitrogen dosage shown in the contour graph is commonly referred to as “baseline skew”. Skew reflects asymmetry of the plasma and may be a result of multiple factors either inherited from the chamber or contributed by the process recipe, for example, asymmetry of the coils, flow rate distribution, chamber structure, species in the processing gas, changes of flow rate, power level of RF source applied, and other external factors from the processing chamber.
Therefore, there is a need to have a plasma reactor with a capacity to correct degrees of baseline skew during process.